The present invention relates to a semiconductor strain sensor for a pressure-detecting apparatus or the like and a method for manufacturing the same.
In a pressure-measuring sensor of the type which is frequently used, a thin diaphragm is formed in a semiconductor substrate, such as silicon (Si), by forming a blind hole in a major surface of the semiconductor substrate and resistance layers made by diffusing an impurity of a conductivity type opposite that of the diaphragm. In the strain sensor of this type, pressure measurement is performed by transducing to an electrical signal the pressure difference between pressures applied to two sides of the diaphragm by way of piezoresistive changes. The semiconductor strain sensor must ideally generate a linear output voltage in proportion to the pressure difference. However, in practice, the relationship between the pressure difference and the output voltage is not completely linear but nonlinear. Further, a change in output voltage for a positive pressure difference must ideally be symmetrical with that for a negative pressure difference. However, when a diaphragm is flat, changes in output voltages for positive and negative pressure differences become asymmetrical. This is because distortions in the diaphragm for the positive and negative pressure differences differ slightly from each other. The asymmetrical characteristic of the output voltages is a cause of degradation in measuring precision. In order to perform a highly precise pressure measurement, the asymmetrical characteristic described above must be compensated. For this purpose, a special correction circuit must be used, and adjusting operations must be performed to compensate for the asymmetry. Furthermore, the asymmetrical characteristic must be compensated for the expense of sensitivity, and the gain of an amplifier circuit system must be increased, resulting in unstable circuit operation.
A semiconductor strain sensor has been proposed wherein a projection is formed on a bottom surface of the blind hole (upper surface of the diaphragm) of the semiconductor strain sensor, with the thickness of the central portion of the diaphragm greater than that of the edge portion in order to improve the asymmetrical characteristic described above. This semiconductor strain sensor is manufactured in the following manner.
As shown in FIG. 1, an annular Si.sub.3 N.sub.4 resist film 102 is deposited on one major surface of a disc-shaped monocrystalline silicon substrate 100. Diffusion resistance layers 104 are formed on the other major surface of the monocrystalline silicon substrate 100. This silicon substrate 100 is fixed by wax 108 on a crystal base 106 such that the other major surface of the monocrystalline silicon substrate contacts the surface of the crystal base. The monocrystalline silicon substrate 100 is dipped together with the crystal base 106 in an etching solution 112 contained in an etching tank 110. The etching tank 110 is supported by a support 114 which is inclined by a predetermined angle with respect to the horizontal direction. The etching tank 110 is coupled through a shaft 116 to a motor disposed in the support 114. The etching tank 110 rotates at a predetermined speed to etch only the central portion of the monocrystalline silicon substrate 100 which is exposed from the resist film 102, thereby forming a blind hole. Since the etching tank 110 is inclined in a horizontal direction, the etching rate at the edge portion of the blind hole is higher than that at the central portion of the bottom surface thereof. As a result, a diaphragm with a concentrically mound central portion is formed.
The semiconductor strain sensor manufactured by the above method has the following drawbacks. The etching rate is very low, degrading working efficiency. In addition to this drawback, since the blind hole is formed by utilizing a difference between the etching rates of the resist film and the monocrystalline silicon substrate, the depth to which the blind hole can be formed is limited to about 300 .mu.m. Therefore, a thick monocrystalline silicon substrate cannot be used, and the ratio of the diaphragm thickness to the monocrystalline silicon substrate thickness is about 1:2 to 1:4. As a result, when the semiconductor strain sensor is adhered to a base through glass, the diaphragm and the diffusion resistance layers are subjected to thermal strain, thereby degrading sensitivity of the strain sensor and decreasing the yield. Furthermore, since wet etching is dependent upon the concentration, temperature and convection state of the etching solution, working precision and reproducibility are poor. In addition to these disadvantages, the corner between the inner surface and the bottom surface of the blind hole becomes rounded, thus further degrading sensitivity of the strain sensor.